CA1276810C - Phase reading fiber optic interferometer - Google Patents

Abstract

PHASE READING FIBER OPTIC INTERFEROMETER
Abstract A system and method for detecting the influence of selected forces on an interferometer over an extended dynamic range. One presently preferred embodiment is disclosed for detecting rotation of an interferometer. In this embodiment, an open-loop, all-fiber-optic gyroscope provides an output signal comprising the phase difference of two light waves which are counterpropagating within the gyroscope, and which are phase modulated at a selected frequency. The phase difference of the light waves is influenced by the rotation rate of the interferometer.
The output signal is amplitude modulated at the phase modulation frequency to transpose the optical phase shift into a low frequency electronic phase shift, which is measured using a digital time interval counter. A linear scale factor is achieved through use of this system and method.

Description

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PHASF~ READING FI~R OPTIC INTERFERO~TER
Back~round of the Invention The present invention relates to interferometers, and particularly to a phase readin~ all fiber optic interferometer which includes a method and apparatus for measuring phase difference signals from the interferometer over an extended dynamic range of operation.
Interferometers typically comprise devices which provide for the propagation of two interfering light waves, with the phase difference between the light waves being dependent upon the difference in the respective optical path lengths traveled by the two light waves within the interferometer. The phase diference between the two interEering waves can also be influenced by ; 15 external forces such as rotation of the interferometer.
Thus, interferometers generally provide an output signal whose intensity is dependent upon the phase difference between the waves. Various methods and devices for detectin~ and measuring this phase difference have been ~`;` 20 devised, but each has been shown to have problems or limitations under certain operatin~ conditions.
evices for measuring t~e phase difference have often found application in interferometers which are used for rotation sensing, Thus, althou~h the method and apparatus disclosed herein for detectin~ and measurin~ the p~ase difference OUtpllt signal is useable with all conventional .
~ interferometers, its configuration and operation may best ~ .
` ~ ~ be described in connection with fiber optic rotation sensors, which comprise one preferred embodiment of the invention~
Fiber optic rotation sensors typically comprise a loop of fiber optic material to which li~ht waves are coupled for propagation around the loop in opposite directions.
Rotation of the loop creates a relative phase difference between counter-propagatinX waves, in accordance with the well known "Saxnac effect", with the amolmt of phase ~ .' ,,;~ ~,~

difference corresponding to the velocity of rotation. The counter-propagatin~ waves, when recombined, interfere - constructively or destructively to produce an optical OtltpUt signal which varies in intensity in accordance with the rotation rate of the loop. Rotation sensing is com~only accomplished by detection of this optical output signal.
Various techniques have been devised to increase the sensitivity of fiber optic rotation sensors to small rotation velocities. For example, one open-loop technique involves phase modulating the counter-propagating light waves at a irst harmonic frequency. The rotation rate may then be determined by phase sensitive detection of a component in the optical output signal at the phase modtllation frequency. The amplitude of this component is proportional to the rotation rate. However, this technique is not available for detecting large rotation rates because the optical output signal defines a wavefor~
~` which repeats itself periodically as the rotation rate increases or decreases. Thus, the amplitude of the measured component is the same at each periodic repetition of the OUtpllt si~nal, even though the associated loops rotation rate is different~ In addition, the sensitivity of the device becomes essentially zero at some locations ; ~5 on the repeatinp~ signal waveform, causing a nonlinear response of the device. Such techniques are difficult to use in many applications requiring rotation sensin~ over an extended dynamic range.
Another technique which involves an open-loop configuration involves a single sideband detection scheme such as the one described in D. Eberhard and E. Voges, "Fiher Gyroscope with Phase-Mo~ulated Single-Sideband Detection," Opt. Lett. 9, 22 (1~84). ~owever, this approach is not feasi~le since it requires a wide band 3~ phase modulator which is not presentlv available in fiher-optic form.

Still another approach to rotation sensing involves a si~nal processing technique, as c1escribed in K. Bohm, P.
~larten, E. Weidel, and K. Reterman, "Direct Rotation-Rate Detection With A Fiber-Optic Gyro By Using Digital Data -~ 5 Processing," Electron. Lett. 19, 997 (1~83). In this approach, like the technique described above, the counter-propagating waves are phase modulated at a selected frequency. An odd harmonic and an even harmonic oE the output signal are each measured, and these signals are ; 10 processed and combined to define the tangent of the phase shift caused by rotation of the loop. The rotation rate may then be calculated from this in~ormation. Because of the limited range of presently available analo~ to digital converters which are used with this device, the device cannot provide the necessary dynamic range at the resolution which is required in many gyroscope ~; applications, such as many types of navigationO
In order to overcome some of the problems associated with the techniques described above, various other closed 2~ loop approaches have been developed. For example, several closed loop techniques include phase modulation of the counter-propa~ating light wa~es at a selected frequency.
The optical output signal produced by the light waves is monitored to detect rotation of the loop. When rotation is detected, a feedback signal is produced which controls -~ the phase modulation sional which is applied to the ~; counter-propagating light waves. In response to the feedback sionalj the amplitude of the phase modulation signal is adjusted to null ou~ the component in the ~ 30 optical output signal produced by the loop rotation.
; Thus, the amplitude of the phase modulation sio,nal ~ comprises a measure of the loop's rotation rate.
-~ These closed loop techniques provide the same sensitivity level which is available in open-loop devices, while also si~nificantly increasing the dynamic range over which t~e rotation rate may be accurately measured.
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However, the precision and range of these rotation sensors is limited in application by the capabilities of the various output devices to which the sensors may be connected. For example, the output devices must have a range and resolution which permits measurement of the amplitude of the phase modulation signal for large, as well as very small rotation rates. Output devices to be used with these systems are not presentLy available with both sensitivit,y levels and dynamic ranges which approach the requirements for applications such as aircraft navig~tLon. In additio-n, these systems are inherently more complex than the open-loop systems due to the additional electronic circuitry included therein.
In light of the above, it would be a great improvement in the art to provide an open-loop rotation sensing system and method wherein the rotation rate of an all-fiber-optic' gyroscope could be precisely, unambiguously and linearly determined over an extended dynamic range. It would be a, further important improvement to provide such a system and method which would utilize presently existing compo,nents to produce digital readout of the rotation rate over a substantially unlimited dynamic range.
Brief Summary of the Invention The present invention comprises an apparatus and method for detecting and measurin~ the influence' of selected external forces on an interferometer over an extended dynamic range of operation. The apparatus includes an interferometer having a detector for providing an output comprised of two interferin~ light waves r~hich propagate within the interferometer. The phase difference between the interferin,~ light waves is dependent upon the difference in the respectitJe optical path len~ths traveled by the li~Jht waves within the interferometer, and upon the influence of e~ternal forces such as pressure, temperature and rotation of the i-nterferometer. The intensity of the output is dependent upon the phase diference of the lioht .

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waves, an-1 thus is also dependent upon the external forces applied to the interferometer.
A first circuit amplitude modulates the output to produce a first signal having selected harmonics which contain both sine and cosine components of the output. A
second circuit functions in response to the first signal to provide a second signal which is representative of shifts in the phase difference of the interfering liRht waves caused by the external forces.
tO In one preferred embodiment, the invention comprises an open-loop rotation sensor and method of its operation for use in accurately and reliabl~ sensing a broad ran~e of rotational velocities and providin~ a phase of a low frequency siRnal which corresponds to the sensed rotation. The rotation sensor comprises all fiber optic components, such as a fiber optic directional coupler' which (a) splits the light from the source into two waves that propagate around the sensing loop in opposite directions, and (b) combines the counter-propagating waves to provide an optical output signal. Proper polariz,ation of the applied light, the counter-propagating waves, and the optical output signal is established, controlled, and maintained by a fiber optic polarizer and fiber optic polarization controllers. A second fiber optic coupler is provided to couple the optical output signal from'the continuous strand to a photodetector which outputs an electrical signal that is proportional to the ~ntensity of the optical si~nal.
Improved operating s~ability and sensitivity of the rotation sensor is achieved by phase modulating the counter-propagatin~ waves at a selected ,frequency throu~h use of a phase modulator, and thereby biasing the phase of -the optical output signal. The optical intensity output signal fro~ the photodetector is amplitude modulated at the phase modulation frequency to transpose the optical phase shift into a phase shift of a low frequency .
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electronic signal. The modulated signal is filtered to select one of its l-.armonic frequencies.
; In one preferred embodiment, the amplitude modulation is accomplished by an electronic switch which alternately transmits the amplitude modulated si~nal to one of two channels at the phase modulation frequencyO Thus, -the signals in the two channels are square wave modulated at the modulation frequency, and are 180 out of phase with each other. A component of each of the signals in the channels is selected at a harmonic o~ the modulation frequency by band pass filters, and the phase difference of these components is determined in a phase meter. This phase difference comprises a value of two times the phase difference produced in the counter-propa~ating waves by the rotation rate. In one preferred embodiment, the phase ~-~ meter is a time interval counter which produces a very - accurate digital output signal which may be readily used in conventional digital output devices.
In another preferred embodiment, the intensity output signal is amplitude modulated by a conventlonal electronic gate, and the phase of a selected harmonic of the amplitude modulated signal is measured against that of a ` corresponding harmonic of the phase modulation signal.
This measurement may be made in a phase meter as described 2S above to produce a digital value corresponding to the phase difference produced in the counter-propagatin~ waves by rotation of the loop. Op~ionally, amplitude modulation could be performed in the optical loop throu~h use of an optical gate such as a shutter.
These and other objects and features of the present invention will become more -full~ appare~t from the following description and appended claims taken in conjunction with the accompanyin~ drawin~s.
Brief Description of the ~rawings FI~1URE 1 is a schematic drawin~ of a basic rotation ; sensor, showing the fiber optic components positioned . ~ .

alon~ a continuous, 1minterrupted strand of fiber optic material, and fur-ther showing the signal generator, photodetector, lock-in amplifier, and display associated with the detection system;
FIGURE 2 is a sectional view of one embodiment of a : fiber optic directional coupler Eor use in the rotation ~, sensor of Figure 1;
FIGURE 3 is a sectional view of one embodiment of a fiber optic polarizer for use in the rotation sensor of : 10 Figure 1;
FIGURE 4 is a perspective view of one embodiment of a fiber optic polarization controller for use in the rotation sensor o Figure 1;
FIGIJRE 5 is a schematic diagram of the rotation sensor of Figure 1 with the polarizer, polarization controllers, and phase modulator removed therefrom;
: FIGURE 6 is a graph of the intensity of the optical output si~nal, as measured by the photodetector, as a function of the rotationally induced Sagnac phase ;~ 20 difference, illustrating the effects of birefrin.gence ::~ induced phase differences and birefringence induced :- amplitude fluctuations;
FIGURE 7 is a graph of phase difference as a function of time showing the phase modulation of each of the :~ 25 counter-propagating waves and the phase difference bet'ween the counter-propagating waves;
FIGURE 8 is a schematic drawin~ illus-trating the effect of the phase modulation upon the intensity of the : optical output signal, as measured by the detector, when 3~ the loop is at rest;
: FIG~JRE 9 is a schematic drawing showing the effect of the phase modulation upon the intensity of the optical 011tpUt si~nal as measured by the detector when the loop is rotating;
FIGU~E 10 is a diagram o~ one preferred embodiment of an open-loop phase reading rotation sensor with extended d~Jnamic range;

FIGllRE 11 is a diagram illustrating the first through ; fourth harmonic frequency components of the optical output - signal from the fiher-optic loop associated with the rotation sensor;
FIGURE 12 is a diagram illustrating the relationship of phase and amplitude modulation signals with switch and filter output signals during conditions of both rest and rotation of the optical loop associated with the rotation sensor illustrated in Figure 10;
- ~ 10FIGURE 13 is a graph illustrating the linear scale factor of the rotation sensor illu~strated in Figure 10;
FIGURE 14 is a diagram o another preferred embodiment of an open-loop, phase reading rotation sensor with extended dynamic range; and 15FIGU~E 15 is a dia~ram illustrating the relationship of phase and amplitude modulation signals with ~ate and filter output signals during conditions of both rest and rotation of the optical loop associated with the rotation sensor illustrated in Figure 14.
` 20Detailed ~escription of the Preferred Fmbodiment As was indicated above, the invention can best be described by reference to its use in conjunction with a par~icular type of interferometer in a particular application which comprises one preferred embodiment of 25the invention. Thus, the invention is described in connection with a Sa~nac interferometer for rotation sensing~ However, it will be appreciated that the invention can be used with any interferometer in many types of applications.
30Before proceeding with a discussion of one preferred ~` embodiment of the invention, a discussion of the basic ~` rotation sensor used in the invention is necessary for a fuller understanding of the i~provement. Figure 1 shows a rotation sensor having a basic optical loop structure which is of the type use~ in the present invention. ~ther components not in the loop are included only for purposes .

of providing one example of how such systems generally are operated. This rotation sensor includes a light source 10 - for introducing light into a continuous length or strand of optical fiber 12, a portion of which is wound into a sensing loop 14. As used herein, the reference numeral 12 designates generally the entire continuous strand of optical fiber, while the numeral 12 with letter suffixes (A, B, C, etc.) designates portions of the optical fiber ; 12.
In the embodiment shown, the light source 1~ comprises a galium arsenide (GaAs) laser which produces light having a wave length on the order o~ 2 microns. By way of speciEic example, the light source 1n may comprise a model GO-DIP laser diode, commercially available from General Optronics Corp., 3005 Hadley Roadj South Plainfield, New Jersey. The fiber optic strands such as the strand 12 are preferably single mode fibers having, for example, an outer diameter of 80 microns and a core diameter of 4 microns. The loop 14 comprises a plurality of turns of the fiber 12 wrapped about a spool or other suitable ~`~ support (not shown). By way of specific example, the loop 14 may have approximately 1000 turns of fiber wound on a form having a diameter of 14 centimeters.
Preferably, the loop 14 is wound symmetrically, starting from the center, so that symmetrical points in the loop 14 are in proximity. It is believed that this reduces the environmental sensitivity of the rotation :~ :
sensor, since such symmetry causes time varying temperature and pressure gradients to have a similar effect on both of the counter-propagating waves.
Light from the source 1~ is optically coupled to one end of the fiber 1~ by butting the fiber 1~ aRainst the light source 1 n. Various components for guidin~ and processino the Light are positloned or formed at various locations along the continuous strand 12. For the purpose of describing the relative locations of these co~ponents, : .:
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-ln-the continuous fiber 12 will be described as being divided into seven portions, la~elecl 12A throu~h l~G, respectively, with the portion 12A through 12E bein~ on the side of the loop 14 that is coupled to the source 1 n, and the portions l?F and 12G being on the opposite side of the loop 14.
Adjacent to the light source 10, between the fiber portions 12A and 12B, is a polarization controller 24. A
type of polarization controller suitable for use as the controller 24 is described in detail in United States Patent No. 4,3~9,090, issued June 21, 19~3, entitled "Fiber Optic Polarization Converter", assi~ned to the assignee of the present invention. A brief description oE the pol~rization controllers 24 will be provided suhsequently. However, it should be presently understood that this controller 24 permits adjustment of both the state and direction of polarization of the applie~ ht. , The fiber 12 then passes throu~h ports labeled A and B
~0 of a directional coupler 2fi, located between the ,fiber - portions 1?,P, and 12C. T~e coupler 26 couples optical power to a second strand of optical fiber which passes throu~h the ports labeled C and D of the coupler 26, the port C bein~ on the same side of the coupler as the port A, and the port D being on the same side of the coupler as the port B. The end of the fiber 2~ extendino from the port D terminates nonreflectively at the point laheled "MC" (for "not connected") while the end of the fiber 29 extendin~ from the port C is opticallv coupled to a photodetector 3n. BY way of speci'ic e~ample, the photodetec~or 3n may comprise a standard, reverse biased, silicon, PIN-t~J?e, photo diode. The coupler 27 is described in detail in Bergh et al, "Single Mode Fiber Optic Direction Coupler", ELECTRONIC LETTERS, Vol. 16, No. 7 (March 27, 1980), and in European patent application Serial No. 823~4705.~, ~iled September 8, 1982 entitled "Fiber Optic Rotation Sensor Utilizing Unpolarized Light" which was published on March 23, lg83 as Publication No. 0074789, as well as in U.S. Patent No. 4,493,528 issued January 15th, 1985, ` entitled "Fiber Optic Directional Coupler", and corresponding to European Patent Application Serial -~ No. 81102667.3 filed April 8, 1981 and published October 21, 1981 as Publication No. 0038023.
The fiber portion 12C extendin~ from port B of the coupler 2fi passes through a polarizer 32, loca~ed between the fiber portions 12C and 12D. A monomode optical fiber has two polarization modes of travel for ~n~ ht wave.
1S The polarizer 32 per~its passage of li~ht in one of the polarization modes of the fiber 12, while preventing passage of li~ht in the other polarization mode.
Preferablv, the polari~ation controller 24 ~entioned above is used to adjust the polarization of the applied Ii~ht so 1 20 that such polarization is substantially the same as the polarization mode passed by the polarizer 32. This reduces the loss of optical power as the applied light ; propagates throuRh the polarizer. A preferred type of polarizer for use in the present invention is described in detail in United States Patent No. 4,386,~2~, issued June 7, 19~3, enti~led "Polarizer and Method", assigned to the assi~nee of the present invention.
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After passing throu~n the polarizer 32, the fiber 12 passes through ports labeled A and B of a directional coupler 34, loca~ed bet-~een the fiber portions 12~ and . Tnis cou?ler 3~- is preferablv of the sa~e type as desc~i~ed above in ref2rence to the coupler 26. The fiber `~ 12 is the~ wound in~o che loop lL, with a polarization controller 3~ locaLed het~een the loop 1~ and fiher I
portion 12~. Tllis ~olarization controller 3~ mav he of , L2~

the type discussed in reference to the controller 24, and is utilized to ad,just the polarization of the li~ht waves ~, counter-propagating through the loop 14 so that the optical output signal, formed by interference of these co~mter-propagating waves, has a polariæation which will be efficiently passed by the polarizer 32 with minimal optical power loss. Thus, by utilizing both the polarization controllers 24 and 36, the polarization of the light propagating throuF,h the Eiber 12 may be adjusted for maximum optical power output.
A phase modu'lator 38 driven by an ~C signal generator 40 is mounte~i in the fiber segment 12F between the loop 14 and the second directional coupler 34. This modulator 38 ;~ comprises a PZT cylinder, around which the fiber 12 is wrapped. The fiber 12 is bonded to the cylinder so that when it expands radially in response to the modulating' signal from the generator 40, it stretches the fiber 12.
An alternative type of modulator (not shown), suitable for use with the present invention, comprises a P7.T
;'' 20 cylinder which longitudinally stretches four se~ments of the fiber 12 bonded to short lengths of capillary tubing at the ends of the cylinder. Those skilled in the art will recognize that this alternative type of modulator may impart a lesser degree of polarization modulation to ,the '~ 25 propagating optical signal than the modulator 3~; however, ` it will be seen subsequently that the ~odulator 3~ may be operated at a frequency which eliminates the undesirable effects of polarization modulation. Thus, either type o ~, ~ modulator is suitable for use in the present invention.-'~ 30 The fiber 12 then passes through ports labeled C and D
of the coupIer 34, with the fiber portion t'2F extending ~ from the por~ n and the fiber portion 12G extendin~ from - the port C. Fiber portion 12G terminates nonreflectively at a poin~ labeled "MC" (for "not connected"~.
~,; 35 The Olltp11t signal fro~ the AC generator 4n is supplied on a line 4' to a lock-in amplifier 46 as a reference .

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signal, which lock-in amplifier 46 also is connected to receive the output of the photodetector 30 by a line 48.
This signal on the line 44 to the amplifier 4h provides a reference slgnal for enabling the amplifier 4~ to synchronously detect the detector output signal at the modulation frequency, i.e~, the first harmonic component ~; of the optical output signal, of the modulator 38 while blocking all other harmonics of this frequency.
Lock-in ampliEiers are well known in the art and are commercially available, It will be seen below that the magnitude of the irst harmonic component of the detector output signal is proportional through a certain limited operating range to the rotation rate of the loop 14. The amplifier 46 outputs a signal which is proportional to this first harmonic component, and thus provides a direct indication of the rotation rate, which may be visually displayed on a ~isplay panel 47. However, the scheme of detection shown in Figure 1 is designed for detection of relatively small rotation rates as will be seen in connection wit~ the discussion of Figure 9.
The Cou~lers 26 and 34 A preferred fiber optic directional coupler for use as the couplers 2~ and 34 in the rotation sensor or ~yroscope ;~ 25 of the present invention is illustrated in Figure 2. The coupler comprises two optical fiber strands labeled 5QA, 50B in Figure 2, of a single mode fiber optic material ~aving a portion of the claddin~ removed from one side thereof. The two strands 50A and 50B are mounted in respective arcuate slots 52A and 52B, formed in respective bloc~s 53A and 53B. The strands 5~A and 50B are positioned with the portions of the strands where the cladding has been removed in close spaced relationship, to form a re~ion of interaction 54 in which the light is transferred between the core portions of the strands. The - amount of ma~erial removed is such that the core portion .

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of each strand 50A and 50B is within the evanescent tield of the other. The center-to-center spacing between the strands at the center of the coupler is typically less than about 2-3 core diameters.
It is important to note that the light transferred - between the strands at the re~ion of interaction 54 is directional. That is, substantially all of the light applied to input port A is ~elivered to the output ports B
and D, without contra-directional couplin~ to port C.
Likewise, substantially all of the light applied to input port C is delivered to the output ports B and D~ Further, this directivlty is symmetrical. Thus, light supplied to either înput port B or input port D is delivered to the output ports A and C. Moreover, the coupler is essentially nondiscriminatory with respect to polarizations, and thus preserves the polarization of the coupled light. Thus, for example, if a light beam having a vertical polarization is input to port A, the li~ht coupled from port A to port D, as well as the light 2~ passing straight through from port A to port B,~ will remain vertically polarized.
~ From the foregoing, it can be seen that the coupler ; may function as a beam-splitter to divide the applied light into two counter-propagating waves W1, W2 (Figure 1). Further, t~e coupler may additionaLly function to ~; recombine the counter-propagating waves after they have ~`~ traversed the loop 14 (Fi~ure 1).
In the embodiment shown, each of the couplers ~6, 34 ~ has a coupling efficiency of fifty percent, as this choice -~ 30 of coup~ing efficiency provides ma~imum optical power at the photodetector 30 (Figure 1). As used herein, the term "coupling efficiency" is defined as the power ratio of the ; coupled power to the total output power, e~pressed as a percent. For example, referring to Figure 2, if ligh~ is applied to port A, the coupling efficiency would be equal to the ratio of the power at port D to the sum of the .~ .

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power output at ports B and D. Further, a coupling efficiency of 5~/0 for the coupler 34 insure~ that the counter-propagating waves W1, W2 are equal magnitude.
The Polarizer 32 A preferred polarizer for use in the rotation sensor o the present invention is illustrated in Flgure 30 This polarizer includes a bireringent crystal 6n, positio-ned witllin the evanescent field oE light transmitted by the fiber 12. The fiber 12 is mounted in a slot ~2 which opens to the upper face h3 o a generally rectangu,lar quartz block 64. The slot 62 has an arcuately curved bottom wall, and the Eiber is mounted in the slot fi2 so ; that it follows the contour of this bottom wall. The upper surface 63 of the block 64 is lapped to remove a portion of the cladding from the fiber 12 in a region 67. The crystal 60 is mounted on the block 64, with the' ~; lower surface 68 o the crystal facing the upper surface 63 of the block 64, to position the crystal 60 within the, evanescent field of the fiber 12.
'~' 20 The relative indices of refraction of the fiber 1,2 and the birefringent material 50 are selected so that the wave velocity of the desired polarization mode is greater in the birefringent crystal 60 than in the fiber 12, while the wave velocity o an undesired polarization mode is -~ 25 greater in the fiber 12 than in the birefringent crystal 60. The light of the desired polarization mode remains guided by the core portion of the fiber 12, whereas lig~t of the undesired polarization mode is coupled from the fiber 12 to the bireringent crystal 6~. Thus, the polarizer 32 per~its passage o light in one polarization mode) while preventing passage o light in the other ~'~` polarization mode. As previously indicated, the polarization controllers 24, 3~ (Figure 1) nlay be utilized to adjust the polarizations of the applied light and ~,~ 35 opti~al output signal, respectively, so that optical power loss through the polarizer i3 minimized.

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-1h-The Polarization Controllers 24 3 , One type of polarization controller suitable for use in the rotation sensor of the present invention is illustrated in Figure 4. The controller includes a base 70 on which a plurality of upright blocks 72A -throu~h 72D
are mounted. Between adjacent ones of the blocks 72, spools 74A throu~h 74C are tangentially mounted on shafts 76A throuRh 76C, respectively. The shafts 76 are axially aliP,ned with each other, and are rotatably moun~.ed between the blocks 72. The spools 74 are ~enera].ly cylin-lrical and are positi.onetl tangentially to the shafts 76.
The strand 12 extends through axial bores in the shafts 76 and is wrapped about each of the spools 74 to form three coils 78A through 7(~C. The radii of the coil 78 are such that the fiber 12 is stres~sed to form a birefringent mediu~ in each of the coils 78. The three coils 78A through 78C may be rotated independently of each other about the axis of the shafts 74A through 74C
respectively to adjust the birefringence of the fiber 12 and, thus, to control the polarization of the l.ight passing through the fiber 12.
The diameter and number of turns in the coils 78 are such that the outer coils 78A and C provide a spatial delay of one-quarter wave :length, while the central coil 78D provides a spatial delay of one-half wave length. The quarter wave length coils 78A and C control the elipticity of the polarization, and the half wa~e length coil 7~
controls the direction of polarization. This provides a full range of adjustment of the polarization of the li~ht propagatinR throu~h the fiber 12.
It will be understood, however, that. the polarization controller may be modified to provide only the two quarter wave coils 78A and C, since the direction of polari~ation (otherwise provided by t~.e central coil 78B) may be controlled indirectly through proper adjus~ment oE tlle elipcicity of polari~ation by means o:E the two quarter wave coils 7~A and C. Accordingly, the polarization controllers 24 and 36 are shown in Figure 1 as including only the two quarter wave coils 7~A and C. Since this configuration reduces the overall size of the controllers 24-36, it may be advantageous for certain applications of the present invention involving space limitations.
Thus, the polarization controllers 24 and 36 provide means for establishing t maintaining and controlling the polarization o both the applied light and the counter-propa~ating waves.
Operation ~1ithout Phase Modulation -_r Polarlzation Control In order to fully understand the Eunction and importance of the polarizer 32 (Fi,~ure 1) and phase modulator 38, the operation of the rotation sensor of Figure 1 will first be described as if these components had been removed from the spstem. ~ccordingLy Figure 5 shows the rotation sensor of Figure 1 in schematic block diagram form, with the modulator 38, polarizer 32, and associated components removed therefrom.
Light is coupled from the laser source 10 to the fiber 12 for propagation therein. The light enters port A of the coupler 26, where a portion of the light is lost through port ~. The remaining portion of the light -~ 25 propagates from port B to port A of the coupler 34, where ~; it is split into two counter-propagating waves W1, IJ2 of -~ equal amplitude. The wave W1 propagates fro~ the port B
~ in a clockwise direction about the loop 14, while the wave - ~2 propagates from port D in a counter-clockwise direction around the loop 14, ~- After the waves IJ1, W2 have traversed ~he loop 14, !

they are recombined hy the coupler 34 to form an optical output signal, which propa~ates from port A of the coupler 34 to port B of the coupler 26. A portion of the optical output signal is coupled from port B to port C of the coupler 2~ for propa~ation along the fiber 29 to the ~L~7~'0 -18~
photodetector 30. This photodetector 3~ outputs an electrical signal which is proportional to the intensity of the light impressed thereon by the optical output signal.
The intensity of the optical output signal will vary :: in accordance with the amount and type, i.e., constructive or destructive, of interference between the waves W1, W2 when they are recombined or interfered at the coupler 34. Ignoring, for the moment, the effects of fiber birefringence, the waves W1, W2 travel the same optical path around the loop 14. Thus, assuming the loop 14 is at rest, when the waves W1, W2 are recombined at the coupler 34, they wil.l interfere constructively, with no phase difference therebetween, and the intensity of the optical output signal will be at a maximum. However, when the.
loop 14 is rotated, the counter-propagating waves W1, W2, . will be shifted in phase in accordance with the Sagnac effect, so that when they are superposed at the coupler 34, they destructively interfere to reduce the intensity : 20 of the optical output signal. Such Sagnac phase difference between the waves W1, W2, caused by rotation of ~- the loop 14, is defined by the following relationship:

~R :~c Q ( 1 ~' 25 `
where:
:~ . A is the area bounded by the loop 14 of optical fiber;
N is the nu~her of turns of the optical fiher ~ about the area A;
: :~ Q is the angular velocity of the loop about an a~is wh:icll is perpendicular to the plane of the loop;
and and c are the free space values of the wave len~th and velocity, respectively, of the li~ht applied to the loop.

_19_ The intensity of the optical out?ut sional (IT) is a function of t~e ~a~nac phase ~ifference (~R) betwe~n t~e ~aves Wl W2, and is de ined b~ the followin~ equation IT Il + I2 + 2 ~ 2 CS(~R) (~) where l1 and I2 are the in~ividual intensities of the waves Wl, W2, respectively.
Fro~ equations (1) and (2) it may be seen that the intensity of optical output signal is a function of the ro~ation rate (Q). Thus, ~n indica~ion of such rotation rate ~ay be obtained by measurin~ the intensity of the optical output signal, utilizin~ the detector 3n.
; 15 Fi~ure 6 shows a curve sn, which illustrates this relationship between the intensity of the optical output si~nal tIT) an~ the Sa~nac phase difference (~R) be~ween `` the counter-propagatino waves Wl, W2. The curve 80 has the shape o~ a cosine curve, and the intensitv of ~he ~0 optical output signal is at a maximum when the ~a~nac phase difference i9 zero. Where the phase difference between the counter-propa~ating waves Wl, W2 is cau~sed entirely by rotation of t~e loop 1~, the c-lrve sn ~Jill varv symmetricall~ about the vertical axis. However, as ~ .
` ~S discussed in Paulath, et al, "Birefringence and Polarization Effects In Fiber Gyroscopes", APPLIED OPTICS, Vol. 21, No. 10 (May 15, 1982), and in European patent application Serial No. 82902595.6, filed July 29, 1981 and published July 27, 1983 as Publication No. 0084055, with polarized light an additional, nonreciprocal, phase difference between the counter-propagating waves Wl, W2 may be caused by the residual birefringence of the optical fiber 12. This ~
additional nonreciprocal phase difference will not ~!
; occur if completely unpolarized light is used.
,~

:~7~

Birefringence induced phase differences occur because light travelin~ in each of the two polarization modes of the sin~le mode fiber 12 travels at a different velocity. This creates a nonrotationally induced phase difference between the waves W1, W2, which causes the waves Wl, W2 to interfere in a manner that distorts or shifts the curve 80 of Figure 6. Such a shift is illustrated by the curve 82, shown in phantom lines in Figure 6.
10Such birefringence induced, nonreciprocal phase difference is indistinguishable from a rotationally induced Sagnac phase diference, and is dependent on ; environmental factors which vary fiber birefringence, such as temperature and pressure. Thus~ ~iber birefringence is the cause of a major source of error in fiber optic rotation sensors.
O eration With the Polarizer 32 : P
The problem of nonreciprocal operation due to fiber birefringence is solved in the rotation sensor of the ~ .
2~ present invention by means of the polarizer 32 (Figure 1) which, as discussed above, permits utilization of only a single polarization mode. When the polarizer 32 is introduced into the system at the point desi~nated by the reference numeral ~4 in Figure 5, light passing throu~h the polarizer 32 propa~ates into the loop 14 in one ~-selected polarization mode. Further, when the counter-propagating waves are recombined to form the optical output signal, any light that is not of the same polarization as the light applied to the loop is prevented from reaching the photodetector 30, since the optical output signal passes through the polarizer 32. Thus, the optical output slgnal, as it travels from port A of coupler 34 to port ~ of coupler 2~, will have precisel~
the sa~e polarization as the light applied to the loop.

. ~ . . - . , .

Therefore, by passing the input light and optical output siRnal through the same polarizer 32, only a single optical path is utilized, thereby eliminating the problem of birefringence induced phase difference caused by the different velocities of propagation in the two pos~sible polarization modes. That i5, by filterin~ out all light which is transferred from the selected mode to the unselected mode by the birefringence in the fiber, it is possible to eliminate all light waves in the unselected mode which might gain or lose phase relative to the selected mode because of the different velocity of . propagation. Further, it should be noted that the polarization controllers 24, 36 (Figure 1~ may be used to adjust the polarization oE the applied li~ht, and optical output signal, respectively, to reduce optical power loss ~: at the polarizer 32, and thus, ma~imize the signal . intensity at the detector 30.
Operation With the Phase Modulator 3~
: Referring again to Figure 6, it will be seen that, ~ 20 because the curve ~0 is a cosine function, the intensity :: of the optical output signal is nonlinear for small Sagnac : phase differences (~) between the waves W1, W2.
:~ Further, the optical output si~nal intensi-ty is relativel:y insensitive to chan~es in p'nase difference, for small values of ~ Such nonlinearity and insensitivity makes it difficult to transform the optical intensity (IT) measured by detector 3n into a signal indica.tive of the ~ rate of rotation of the loop 14 (via equation 1).
; Further, although birefringence induced phase differences betT~ee~ the waves Wl, W2 are eliminated, as discussed above by use of the polarizer 32,..nevertheless cross coupling between polarization modes caused by fiber birefringence occurs. This cross couplin~ reduces the optical intensity of the optical OUtpllt signal since the cross collpled light is pre~ren~ed from reachin~ the photodetector 30 on the polarizer 32. Thus, changes in ' fiber birefringence cause the amplitude of the curve 80 of Figure 6 to vary, for example, as illustrated by the curve 84. It will be understood that curves ~0, ~2, ~h of Figure 6 are not dra~n to scale.
The foregoing problems are solved in the device of Figure 1 by means of a synchronous detection system utili~.ing the phase modulator 3~, si~nal generator 40 and lock-in amplifier 4fi shown in Figure 1.
Referrin~ to Figure 7, the phase modulator 38 modulates the phase of each of the counter-propagating waves W1, W2 at the frequency of the signal generator : 4n. However, as may be seen from Figure 1, the phase modula~or 38 is located at one end of the loop 14. Thus, the modulation of the wave W1 is not necessarily in phase : 15 with the modulation of the wave W2. Indeed, it is : preferable for proper operation of this synchronous.
- detection system that the modulation of the waves W1, W2 be 1~q out of phase. Referring to Figure 7, it is preferable that the modulation of the wave W1, represented by the sinusoidal curve 90, be 180 out of phase with the ~ modulation of the wave W2, represented by the curve 92.
-~ Use of a modulation frequency which provides such 1~no : phase diiference between the modulation of the wave W1 relative to that of W2 is particularly advantageous in ~5 that it eliminates modulator induced amplitude modulation :~ in the optical output signal measured by the detector : 30. This modulation frequency (fm) may be calculated . using the followin~ equation:

m ~ (3) ~- .
~: ~ where:
L is the differential fiber len~th between the coupler 3h and the modulator 38 for the counter-propagatin~ waves ~1, W2, i.e., the distance, measured . .
'~

~7 E;~

along the fiber, between -the modulator 38 and a symmetrical point on the other side of the loop 14;
neq is the equivalent refractive index for the single mode fiber 12; and c is the ree space velocity of the light applied to the loop 14.
At this modulation frequency (fm) which is called the "proper" frequency, the phase difference (~1) between the counter-propagating waves W1, W2, stemming from phase 1 n modulation of these waves in accordance with the curves sn and 92, is illustrated by the sinusoidal curve 94 in Figure 7. The curve 94 is obtained by subtracting the curve 92 from the curve 90 to obtain the phase difEerence between W1 and W2. This modulation of the phase diference between the waves W1, W2 will also modulate the intensity (IT) oE the optical output signal in accordance ~: with the curve 80 of Figure 6 just as a Sa~nac phase shift would, since such phase modulation Q~1 is indistinguishable from rotationally induced Sagnac phase differences ~R .
The foregoing may be understood more fully throu~h reference to Figures 8 and 9 which graphically illustrate the eEfect of (a) the phase modulation ~ defined by the curve 94 of Figure 7, and (b) the Sagnac phase difference ~R~ upon the intensity (IT) of the opt~cal ~: output signal. Before proceeding with a discussion of Fi~ures ~ and 9, it should first be understood that the intensity (IT) o the modulated optical output si~nal is a .~ unction of the total phase dierence hetween the waves -~ 30 W1, W2. Such total phase diference is comprised o both the rotationally induced Sagnac phase difference ~R and the time varying modulation induced phase difference ~
The total phase diEference ~(~ between the waves W1, W2 -~
may be e~pressed as follows . 35 :
.

P = ~ + ~1 (4) Accordingly, since the effects of the modulation induced phase difference ~ as well as the rotationally induced phase difference ~R will be considered in reference to Figures 8 and g, the horizontal axis for the curve 8Q has been relabeled as ~p to indicate that the total phase difference is being considered, rather than only the rotationally induced phase difference, as in Figure 6.
It will be understood that although the phase difEerence term ~R is used in reference to the preferred embodiment as representing a rotation induced phase difference, this tenm, in a generic sense, represents the phase shift induced by whatever external force or physical quantiLy is being sensed, e.g. rotation, pressure, temperature, etc. Further, those skilled in the art will recognize that Equation (4), and subsequent equations set forth herein, were derived specifically for Sagnac interferometers in which the interferin~ light waves travel the sa~e optical path. If the invention is utilized in another type of interferometer, such as a ` ~ach-Zehnder interferometer, which is structurally configurated to provide different optical paths for the t~ waves, an additional phase term should be added to define the phase shift attributable to the structurally different optical paths. Th2 addition of the ~ 25 furtL1er phase term, however, does not alter the solution for - interferometer phase detection, as is provided by the present invention, nor does it affect the analysis of ~uch phase detection as ~presented herein. ~quation (4), above, and equations (5)-(~), (8)-(9) and (l 1)-tl7) may be revised to re~lect the structure induced phase difference merely by substitutin~ the quantity t~R ~ ~Pst) for ~'PR , where ~Pst is t~e phase shift attributable to the structurally dirferent optical paths. ~quations (7) and (10), on the other hand, require no such revision to account for this structurally induced phase difference ~Pst ' Referring now to Fi~ure 8, the effect of the phase modulation ~Pl (curve 94) upon the intensity IT of the optical .

output si~nal will be discussed. Curve sn represents the relationship between the intensity of the optical OUtpllt signal rest~ting from t~o interfering coherent waves to the phase difference ~ between the waves. When the relative phase angle between them is zero, as illustrated at 93, the resultant intensity of the combined wave is a maximu~, as illustrated at 95. When the relative phase between the waves ~ and W2 is non-zero, the combined optical signal will have a lower intensity dependin~, upon the ma~nitude of the phase difference ~. The intensity continues to t0 decrease with increasing Q~ until the relative phase difference is either plus or minus 18no, as illustrated at 97 and 99 respectively, At a phase di~ference Oe pl~9 or minus 1~n, the two counter-propagating waves completely destructively interfere, and the resultant intensity is zero as illustrated at 97 and 99.
In Figure &, it is assu~ed that the loop 14 is at rest, and thus, the optical signal is not affected by the Sa~nac effect.
Specifically, it may be seen that the ~iodulation induced phase difference curve 94 causes the optical OtltpUt si&nal to vary as ; illustrated by the curve 96. The curve 96 is obtained bY translating the points on the curve 94, representin~ the instantaneous phase ~ifference ~1 between Wl and W2 onto the curve 80 representing the resultant optical intensity for a phase difference of that magnitude. When all the points on the curve 94 are translated onto the curve 80, and the corresponding intensities are plotted, the curve 9h resuIts. The translation of the curve 94 through the curve 80 is symmetrical about the vertical axis of the curve 80, so that - the optical intensity measured by the detector 3n varies periodically at a frequency equal to the second harmonic of the modulating frequency, as shown by the curve 9~.
l~en the loop 14 is rotated~ the counter-propagating waves l~ , W~
are shifted in phase, as discussed abcve, in acco~ance with the Sagnac effect. Ihe Sa~nac phase shift provides a constant phase difference ~ Eor a constant rotational velocity. This Sagnac phase shift adds to the phase difference ~1 created by the modulator 3~, so that the encire cu~Je 94 is translated in pl1ase from the position shown in Fi~lre ~, by an æmount equal to ~R~ as shown in Fi~ure 9. This causes the optical output signal to vary nonsymmetrically along the curve ~0 hetween the points 99 and 101. This causes an optical output signal as illustrated by curve 96.
The points on the curve 96 are derived as follows.
The combined phase difference, ilLustrated at 1~3 on curve 94, translates through the point 101 on the curve sn to the point 105 on the curve 96. The point 107 on the curve 94 translates throu~h the point 109 on the curve 80 to a point 111 on the curve 96. Likewise, the point 113 translates through the point 99 to the point 115, and the point 117 translates through the point 1~9 to the point 119. Finally, the point 121 translates through the point 101 to the point 123.
The optical output signal 96 has a first harmonic component as illustrated in phantom lines of the sinusoidal curve 98. The peak amplitude of the first harmonic component 9~ need not, however, exactly match the amplitude of the optical output signal at point 115 althou~h it mi~ht in so~e cases.
;~, 20 It will be seen subsequently that t.he RMS valuç.of this sinusoidal curve 9~ is proportional to the sine of the rotationally induced Sagnac phase difference Q~R.
Since the amplifier 46 synchronously detects signals having the fundamental frequency of the modulator 3~, the a~plifier 46 will output a signal that is proportional to the RMS value of the curve 980 This si~nal can be used to indicate the rotation rate of the loop.
Tne drawings of Figure 9 illustrate the intensity waveform of the optical output signal for one direction of rotation (e.g., clockwise) of the loop 14. However, it will be understood that, if the loop 14 is rotated in the opposite direction (e.~., counter-clockwise) at an equal velocity, the intensity waveform 96 of the optical output signal will be exactly the same as illustrated in Fi~ure ~, except that it will be translated so that the curve 9 is shiEted 180 from the position shown in Figure 9.

, , : : ',, .', '.'.

~;276~

The lock-in amplifier 4~ detects this 180 phase difference for the curve 98, by comparing the phase of the first harmonic 9~ with the phase of the reference signal from the slgnal generator 4n, to detel~nine whether the rotation of the loop is clockwise or counter-clockwise.
- Depending on the direction of rotation, the amplifier 4h outputs either a positive or negative signal to the display 47. However, regardless of the direction of rotation, the magnitude of the signal is the same for equal rates of rotation of the loop 14.
- It will be recalled from the discussion in reference to Rquation (3) that, by operating at a specific or "proper" frequency at which the phase difEerence between the modulation o~ the waves W1 and W2 is 180, the odd harmonic frequency components of this amplitude modulation, that are induced in each of the counter-propagating waves U1, W2 by the modulator 38, cancel each ~;: other when the waves are superposed to form the~optical output signal. Thus, since the above-described detection ~ 20 system detects only an odd harmonic, i.e., the fundamental ;~ frequency, of the optical output signal, the effects of the undesired amplitude modulation are eli~ninated.
A further benefit of operating at the proper frequency is that even harmonics of the phase modulation, induced by the modulator 3~ in each of the counter-propagating waves ~1, W2, cancel when these waves are superpased to form the optical output signal. Since these even harmonics may, by superposition, produce spurious odd har~onics in the optical signal which might otherwise be detected by the ~;~ 30 detection system, their elimination improves the accuracy -~ of rotation sensing.
; In addition to operating the phase modulator 38 at the -~ ~requency defined bv Equation (3), it is also preferable in the device of Figure 1 ~o adjust the magnitude of the phase modulation so that the amplitude of the detected first harmonic of the optical output ~signal intensity is ,~ .

.
.

maximized, since this provides improved rotation sensing sensitivity and accuracy. It has been found that the ` first harmonic of the optical output si~nal intensity is : at the maximum, for a given rotation rate, when the : 5 amplitude of the modulator induced phase difference ~1 ~:: between the waves W1, W2, indicated by the dimension labeled Z in Figures 7, ~, and 9, is 1.~4 radians. This may be unders~ood ~nore fully through reference to the following equation for the total intensity (IT) of two superposed waves havin~ individual intensities of I1 and I2, respectively, with a phase dif~erence therebetween.
.:
T I1 + I2 + 2 ~I1I2 cos(~) (5) ; 15 h w ere:
~ a~ R + a~1 (6?

`~ 20 ` and ,~
~1 = Z sin(2~fmt), Thus, ~ R + Z sin (2~fmt) he Fourier e~pansion of cosine (~) is:
( ~A) {Jo(Z) + 2~=lJ2n(z)cOs[2~(2nf t)]}
: : :
( ~R) ~2~=1 J2n_1(Z)sin[?~(2n_1)f t1} (9 : ~: 35 :~ .

where Jn(z) is the nth Bessel function of the variable z, and z is the peak amplitude of the modulator induced phase difference between the waves W1, ~12.
There~ore, detecting only the first harmonic of IT
yields:

T(1) 4~ ~ J1(z)sin(~R) sin(~fmt) (10) -~:
`~ 10 Thus, the a~plitude of the first harmonic of the " optical output signal intensity is dependent upon the vaLue oE the first Bessel flmction J1(Z)- Since J1(z) is a maximum when z equals 1.84 radians, the amplitude of the ! phase modulation should preferably be selected so that the t5 magnitude (z) of the modulator induced phase, difference ~1 between the waves 111, W2 is 1.84 radians.
, Reducin~ the Effects of Backscatter As is well known, present state-of-the-art optical ` fibers are not optically per~ect, but have imperfections such as density fluctuations in the basic material of the fiber. These imperfections cause variations in the refractive index of the fiber which causes scattering of small amounts of light. This phenomena is commonly referred to as Rayleigh scattering. Although such scattering callses so~e light to be lost from the fiber, ` the amount of such loss is relatively small, and ~ thereEore, is not a ma,jor concern. ,, `~ The principal problem associated with Rayleigh scattering relates not to scattered li~ht which is lost, but rather to light which is re~lected so that it propagates through the fiber in a direction opposite to its original direction of propa~ation. This is commonly referred to as "backscattered" light. Since such backscattered li~ht is coherent with the light comprisino ~; 35 t~e counter-propa~ating waves Wt, W2, it can ~" constructively or destructively interfere with such ~;' .

; ~2~i81~

propagating waves, and thereby cause variation in the intensity of the optical output signal, as measured b~ the detector 3 n .
~;~ The portion of backscattered light from one wave which will be coherent with the counter-propa~ating wave is that which is scattered within a coherence length of the center of the loop 14. Thus, by reducing the coherence length of the source, the coherence between the backscattered light and the counter-propagatin~, waves is reduced. The remaining portion of the backscattered light will be incoheren~ with the counter-propagating wave, and thus, the interference therebetween will vary randomly so that it is averaged. Therefore, this incoherent portion of the backscattered light will be of substantially~constant intensity, and consequently, it will not cause si~nificant variations in the intensity of the optical output signal.
Accordingly, in the present invention, the effects of backscatter are reduced by utilizing as the light source 10, a laser having a relatively short coherence length, ~` for example, one meter or less. By way of specific example, the light source 10 may comprise the model GO-DIP
laser diode, commercially available from General Optronics Corp., as mentioned above.
An alternative method of prohibiting destructive or constructive interference between the bacl~scattered waves and ~the~propagating waves involves the inclusion of an additional phase modulator in the system at the center of the fiber ~loop 14. This phase modulator is not ;30 synchronLzed with the modulator 3~.
The pro~pagating waves will pass through this additional phase modulator one time only, on their travel around the loop. For backscatter w~ich occurs from a propagatin~ wave before the wave reaches the additional modula~or, the backscat~er will not be phase modulated by this additional modulator, since neither its sollrce .

.
,, , , . , ~ , ...

~27~

.~

` propagating wave nor the backscatter itself has passed ` through the additional modulator.
On the other hand, for backscatter which occurs from a propagating wave after the wave passes through this additional phase modulator, the backscatter will be ;`;~ effectively twice phase modulated, once when the propagating wave passed through the additional phase modulator, and once when the backscatter passed through ~`~ the additional modulator, Thus, if the additional phase modulator introduces a phase shift of ~(t), the backscattered wave originating at any point except at the center of the loop 14 wiLl have a phase shift of either zero, or 2~(t), either of which is time varying with respect to the ~(t) phase shift for the propagating wave. This time varying interference will average out over time, effectively eliminating the effects ;of the backscat~ter.
In yet another alternative method of prohibiting destructive or constructive interference from backscatterj 20 ~ the addl~tional phase modulator, not synchronized with the modulator 3~,;may be introduced at the output of thè light source 10.
In this case, backscatter occurring at any point other than the center of the loop 14 will have a different 2S optical~ path length from the light source 1~ to the detector~ 30 than does the propagating wave from which the backscatter originated.
Thus~, the propagating wave will traverse the loop 14 one time, while the backscattered wave and the propagating ~ wave from which~ it orlglnated will have traversed a portion~of the loop 14 twice. If this portio~ is not one-alf of the loop, the path lengths diE~er.
ecause ~the patb lenoths di~ffer, a propa~atinR wave which reaches the decector 3~ will have been ~enerated at the source 1n at a different time than a backscattered wave which reaches the detector 30 si~1lltaneouslv.
: :~ , , :
: -~ .

~L~r2~
;

The phase shift introduced by the additional phase modulator at the source 1n introduces a phase shift ~(t) relative to the propagatin~, wave, but a phase shift of ~(t+K) to the backscattered wave, where K is the time difference between the passage of the waves through the modulator. Since ~(t+K) is time varying with respect to ~(t), the backscattered interference wilL average out over time, effectively eliminating the effects of the ' backscatter.
Open-Loop Extended Dynamic Ran~e ~etect,ion System The detection system described above with reerence to Figures 1-9 is a very e~ective rotatlon ~ensing system within a certain range of rotational velocities for the ~'~ loop 14. However, the dynamic range is li~ited by certain ~'~ 15 phenomena. For example, with reference to Figure 9 it is, seen that the sensitivity of the detection system can be reduced at very small rotation rates or when the rotation causes the centra] axis of curve 94 to be near points 95 or 97. ,, It can also be seen that the curve 80 is perio,dic.
Thereforej if a large rotation rate causes a large enough ~R to move the central axis of curve 94 past ` either the point 97 or the point 95, then the unction 96 could repeat itself for a second, hi~her rotation rate.
This second rotation rate would be substantially greater than the rotation rate which caused the Sagnac phase shift ~R depicted in~ Figure 9, but the output optical signal 96 could correspond to the one produced at the lower rotation rate. That is, if the ~R from some larger rotational velocity were sufficiently large to move the curve 94 so as to operate between two new points 99' and 101' on the second lobe of the curve ~0, then the itput optical si~nal 96 could appear as it does for the : :

;' .
:' , ~ :

case shown where the curve ~4 operates between the points 9q and 101.
The present invention comprises a novel method, and n associated open-loop apparatus, for extending the range in which the infIuence of external forces on interferometers, ~-~ such as rotation of optical fiber gyroscopes, may be -~ accurately and reliably sensed. The present invention additionally provides a phase of a low frequency signal ~ which corresponds to the effects produced by external `~ 10 forces, such as rate of loop rotation, and which may be ; conveniently utilized for providing data to conventional ~igital output devlces in order to quantiy those efEects, such as rotation rate.
One presently preferred embodiment of applicant's novel rotation sensor is illustrated in Figure 10. It will be noted that the detection syste~ of Fi~ure 10 includes an open-loop fiber optic sensor configuration.
; The detection system of Fi~ure 10 embodies many of the components of the system illustrated in Figure 1. Thus, for purposes of simplicity, those components of Figures ~1 ,; and 1~ which have the same structure and function have `~ been assi~ned corresponding reference numbers.
It has been noted herein that the differential phase shift (~o~) is linearly proportional to the rotation ; 25 rate. Howeuér, the intensity output from detector 3~ is a nonlinear~ (periodic) function of the rotation rate.
Therefore~, in order to obtain extended dynamic range on this open-loop system, it is necessary to recover the original optical phase information from the optical output s~ignal of detector 30.~
In the device of Figure ln, the optical-signal from the rotation;loop is converted into an electrical out?ut signal by detector 3~. This electrical output si~nal contains components a~ the phase moduIation frequency fm and its harmonics, as indicated by the following equation:
~ ~ :
:' ~ :

:: ~
.,~ .

- . , . : . -, . . . ~ -~. . . .. .

I(t) = C[1 + cos~ sin ~mt + ~R)]

= C[1 + {JO(Q~ ) + 2 ~ J2n(~m)cos 2n~mt}cos( ~ ~:
~ {2n_1J2n~ m)sin (2n-1)wmt}sin (~R)] (11 ; 10 Where C is a constant; Jn denotes the n-th order Bessel function; ~m ls the amplitude of the phase difference `~ between the counter-propagating waves produced by the modulation; and wm a 2~fm~
The present invention seeks to overcome many of the problems experienced in the art by providing an open-loop rotation ~sensor wherein the original optical phase information can be utilized to quickly and accurateiy provide a phase of a low frequency signal which is representative of ~R. This could be accomplishe~ if the components of the output signal included two sinusoidal si~nals at the same frequency (n~m) having amplitudes~of cosine ~R~and sine ~R~ respectively, with their phases 25~ ~in quadratu~re. In that situation~ through use of well known trigonometric rules, these si~nals could be added directly to obtain a single, low frequency, sinusoidal signal whose phase corresponds to ~R The present invention~provides such a sin~le sinusoidal signal throu~h ~processing~of ~the output slgnal from detector 30 as described below. ~ ;
Equation (11) indicates that the output from detec~or : 3n contalns terms oE the above kinds, lacking only in that the cosine ~R and sine ~R terms are of diEferent Erequencies- Fi~ure 11 graphicallY illustràtes this ~ relationship as it exLsts between the Eirst through fourth ';
: , . , - . . -. .

:

harmonic fre~uency components of the output sl~nal from detector 3~. Specifically, it is seen that the zero crossings of all odd harmonic frequency components (102,106) correspond to the ~ero crossings of the phase difference modulation signal 1 n2 ~ and that all even harmonic frequency components ~104,1n8) are at their peak : (sno out of phase with the phase modulation signal 1~2) at each zero crossin~ of the modulation signal. The waveforms of these harmonic components can be mathematically defined as follows:
Odd harmonics ~ (2n-l) sin ~mt sin ~R (12) Even harmonics ~ 2n cos ~mt cos ~R (13) where n = an inte~er value Since the various harmonics are at different ` 15 fre4uencies, the above relationships cannot be directly utilized to obtain a single sinusoidal signal whose phase ~ is ~R. However, if the above waveforms existed at the `.'7~ same frequency, then the desired single sinusoidal signal having a phase of ~R could be produced by combining those sinusoidal signals as follows:
~ sin n~mt sin ~R+ C09 n~mt cos ~R = cos(n~mt-~R) (14) ': :

25~ The rotation sensor of Figure 10 comprises one preferred embodlment of a rotation sensor which achieves the above waveshape relationship. Specifically, this relationship~ is achieved in the device of Figure 10 through use of amplitude modulation. Amplitude modulation simply involves making the amplitude of the electrical output signal from detector 30 vary in accordance with the amplitude of a modulating signal.
When the output signal from detector 30 is amplitude modulated b-y a modulating signal having a frequency which is an odd mul~iple of the phase modulation frequency fm (which is also the difference frequency between adjacent .
'' '~

. .
: ~ .
.

:~ ~ f ~ _ harmonics), then each component of the output signal from :detector 30 which is a harmonic of the fm frequency ,~ becomes partially translated into the frequencies of its -~ ~ harmonic nei~hbors. In other words, throu~h amplitude ~; 5modulation in this manner, sideband frequencies are created at harmonics of the phase modulation frequenc~
These sideband frequencies contain sinusoidal components which have been frequency shifted from harmonic amplitude modulated components of the output signal from detector 3n. These si~eband requencies are combined with the component of the OUtp~lt si~,nal at the correspondin~
frequency. Thus, components of the output signal from detector 3n which are harQonics of the fm frequency define waveforms of t:he type defined by equation (14). These and 15other characteristics of amplitude: ~odulation are enerally known to those skilled in the art and are : described~ in detaiL in F.G. Stremler, Introduction to Communication Systems,~(1979) Subject matter of ~:~ particular relevance at this point is set forth on 20pages 191-260 of the Stremler text.
Based:on the above, it will be appreciated that a sinusoidal amplltude mo~ulation at the frequenc~ fm will :transfer:ener~y out of:each harmonic frequency co~ponent 2~5 ~and into:~ the ne~rest har~onic frequenc~ neighbors.
Further,::each si~nal resultin~ from such an amplitude modula,ion ~ill: be in phase with its correspondin~
: harmonic frequency:~ componen~ in the OUtp!lt si~nal~from :detector~3n. The result: o~ such amplitude modulation is ~that all::harmonics then contain terms in both cosine ~R
and sine:~R such that the n-th harmonic has a ter~ coslne -~ (n~m ~ ~R~ Thus, the ~avnac optical phase shift ~R has een isolated and t ans~osed to a low frequencv phase :, shif~ which can he measllred directly by standard means.
~: 35nne e~ample of the use of the detection sensor : illustrate~ in Fi~ure 10 for detectinQ the rotation rate .1 :
" .

~2~
`:

o~er an e~tended dynamic range ~ay be described by reference to Figure 12 in conjunction with Figure 10.
~.~ Specifically, a signal generator 150 (Figure 1n) produces : a phase difference modulation signal at frequenc~ fm :~ 5 having a waveshape as illustrated at 200 in Figure 12, corresponding to sine h~mt~
Preferably, the phase modulation frequency fm corresponds to the "proper" frequency fp which was ~ described previously with reference to equation (3). ~y :~ 10 phase modulating the counter-propagating waves at the `.~ proper frequency, t~e sensitivity of the rotation sensor is greatly improve~. Of course, the sensor wilL also operate at frequencies other than fp, but additional noise and reduced sensitivity will result, as was previously explained.
The phase modulation signal from generator 150 is applied to phase modulator 38, thereby phase modulating : the counter-propagating light waves within the loop in~the~
manner described with reference to the device of Figure ` 20 1- The resuIting optical output signal on flber ~9 is ~s~ detected by a detector 30, which produces an electronicoutput si~nal corresponding to the optical output `: signal. The electronic si~nal from detector 3~ is : amplified in a conventional AC amplifier 152 and : transmitted via line 1 5L to the input of a conventional, double pole switch 1 5 6 .
s~ Switch 156 functions ~in response to a control signal :: re:c.eived : via line 164 from a conventional :electronic sig~al delay circuit 162. In the preferred embodiment, : the control signal is~ at the phasè modulat:ion frequency fm. Cir~cuit ~16~2 is electrically connected to signal : ::; generator 150,:so as to receive the control signaI from : that generator. Delay circui~ 1~2 may be adjusted in order to synchronize the 3igna 1 received from generator 3S 150 to the phase of t~e slgnal received i.n switch 156 from : line 154~
'~

,~

~ - - . . - ' :. ~ , ; ' 71i~

-3~-In res~onse to ~he control signal from delav circuit 1h2, switch 156 transfers the signal from line 154 to one of two output ports 158 and 160 which define, respectively, the input ports of channels 1 and 2 of the detection system. This switching action functions to ~` amplitude modulate the signals received from line 154 at ;~ the fm fre~uency of the synchronizing signal received via line 164 from delay 1~2. The square wavefor~ of the amplitude modulation produced by switch 15fi is graphically illus~ratéd in Figure 12(~) at 2~2 for channel 1, and at 2~4 for channel 2.
It will be appreciatecl that the isquare wave amplitude modulation provided in the device of Figure 1~ is only one ; ~ of many waveforms which could be utilized for this ;`~ 15 amplitude modulation. The square wave modulation merely comprises an embodiment which is particularly simple to im?lement in the device of Fi~jure 1 n. In addition, it will be appreciated that amplitude ~odulation at frequencies other than fm~ or at odd harmonics of fm could ~, ~ 20 be utilized.
; However, due to the trigonometric relationships between the wavefor~s, am?litude modulation at even harmonics of fm would not produce couplin~ between adjacent harmonic frequencies. Rather, amplitude modulation~at even harmonics of fm would resul~ in the even harmonics cou?linv ~wi~h even harmonics, and odd harmonics coupLin~ with odd har~onics. This situation is generally ~understood bY those skilled in the art, and the basis for this condltion mav be more fully understood by reference to~the Stremler text. These problems are avoided if amplitude modulation at the odd harmonics is utilized.
Referr-n~ to Fi~lre 12(C~ i~ is noted that when the loop is not rotatin~, the square wave ampli~ude modulatlon 3S produced b~J s~itc!1 15~ provides an output si~nal 2n~ inchannel 1 which is 18~ out of phase with respect to the : :

~27~8~L~

~ output signal 20~ in channel 2. ~ amplitude modulating -~;; the signal on line 154 to produce si~nals which are l ~()o out of phase, the sine and cosine relationships of the components may be readily evaluated.
From the output 158 of switch 156, the modulated ~-~ signal in channel 1 passes through a band pass filter 166 which is tuned to select one harmonic component (n~m) of the signal in channel 1 (wherein n - the seLec-ted - harmonic). Likewise, the si~nal from the output 160 of switch 156 is transmitted to a band pass ilter 168 which is tuned to select a component oE the signal in channel 2 ~` at the corresponding harmonic frequency. The iltered signals transmitted from filters 166 and 16~ may be mathematically described as follows:

I1 ~ K1 c06(~R) cos(nwmt) + K2 sln (~R) sln (n~mt) Channel 2:
I2 =~ K3 cos(~R) cos (n~t) + K4 sin (~R) sin (n~mt) (15) where K1 through K4 are constants determined by ~m and n.
In the example iLlustrated in Figure 12, the second harmonic of the fm frequenc~ was chosen in order to avoid noise produced by the; electronics, as well as to precludé
spurious~signals~ which may be produced by the switch at 25 ~ ~ the fm ~frequenc~J. ;~f course, it will be appreciated that other harmonics of fm could also be selected, based upon the frequency range d~esired~and the characteristics of the switch and the eLectronic components.
At the~second~harmonLc~ frequency, the constants K
through K4 may be d;e;scribed as fo1lows K1 = K3~ J2(~m) K2 ~K4 ( ~ 1 ) J?n~ b~) / ( 2n -3) ( ?n+l ) ~;
,''; ~ ~

.~ ~

-4~-If K1 = K2 = K, the equation 15 becomes:

.

I1 = K cos(2~mt - a~R) (17) I2 = K cos(2~mt + A~) Those skilled in -the art will recagnize that the evaluation oE the coefficients K1-K~ depends upon several factors inclùding, for example, the amplitude of phase modulation applied to the phase modulator, the waveform of the amplitude modulation, the frequency of switching in switch 15~, and the frequency to which the band pass filters 1~6 and 168 are tuned. Given this informatio~, one skilled in the art can determine the values of K1-K4 by conventional mathematical means, as generally set forth in numerous reference sources such as the Stremler text cited above.
The relationship of Equation (17) may be obtained witho~t use of mathematics by actual adjustment of elements of the rotation sensor. For example, one may 2~5 select the frequency of the switch, the frequency to which the~band pass filters are tuned~ and the waveform of the amplitud~e modulat~ion~. The values of K1 and K2 in Equation (17) can then be made to equal each other by merely adjusting; the amplitude of the phase modulation signal ;30 which is~;applied to modulator 38. In tuning the system to a condi~tion where K1=K2, the amplitude of the phase ; modulation signal is repeatedly adiusted, and the loop is rotated,~ until the amplitude of the signal from the band pass filters does not change as a result of the rotation.
.

., .

:~, .
:, : ' - : ' .

Referring again to Figure 12(C), the 2nd harmonic output waveform fro~ band pass fiLter 166 in channel 1 is illustrated at 210. Likewise, the 2nd har~onic waveform from output band pass filter 168 in channel 2 is illustrated at 212.
The signals (210,212) on lines 170 and 172 of Figure are transmitted into a phase meter 174, which may comprise a conventional time interval counter, such as a Model No. 5345A, manufactured by Hewlett-Packard. In this phase meter 174, the time interval counter is activated as the waveorm 21~ of Figure 12(C) crosses zero, and continues to count untll the waveform 212 crosses zero.
The total count identifies the phase difEerence between waveforms 210 and 212 which corresponds to two times the phase difference ~R. f course, this phase difference is representative of the amount of rotation experienced by the optical loop. Therefore, the phase difference measurement produced by phase meter 174 is representative of the rotation rate of the loop.
The waveforms in Fi~ure 12(C) are produced when no rotation is experienced by the optical loop. Under those~
conditions, the waveforms 210 and 212 are in phase, and the phase meter 174 woul~ therefore detect no phase differénce between those waveforms. This situation is illustrated in Figure 12(C) at 214 where it is seen ~hat the value of ~R is zero. Thus, in this situation, the output from phase meter 174 would also be zero. This signal from meter 174 is then passed onto line 17~ from whence It may be utilized by an~ conventional output device 17~8 such as a digital computer, for communicatin~
the rotatlon~rate of the loop.
Typically, the output device 178 would be capable of maintainin~ ~ a record of t~e most recently measured rotation rate so that information as to present changes in the rotation rate from phase meter 174 would be utilized to update the rotation rate record oE output device 178.
.~:
~ ~ .
.

:~

9L2~6~

Tl~.us, i:E the operatin~ range of the system was such that the zero crossing of signal 212 extended beyond one period from the zero crossing of the waveform 210, the digital output device would determine the rotation rate at this 5 extended dynamic range, even thoug~h the output from the phase meter 174 by itself would not be able to reflect that this measurement was made beyond the first period of . the waveform.
Referring now to Figure 12(D), the condition experienced in the rotation sensor of Figure 10 as a result of loop rota.tion in the amount of 40 per second is illustrated. ~gain, the phase modulation signal is applied at requency fm as indica~ed at 200, and ttle output signal from detector 30 is amplitude modulated in ~: 15 switch 156 with a square wave signal 2n2, 204 at a frequenc~ fiT~- The output signal in channel 1 from switch 156 is illustrated at 216 of Figure 12(D), while the output from switch 156 in channel 2 is illustrated at 218. The corresponding band pass filter output for 20~ channel 1 is illustrated at 22(), while the output for channel 2 is illustrated at 222.
As a result of the rotation, phase meter 174 will detect a phase di:~ference between the waveform 220 of channel 1 and the waveform 222 of channel 2. This phase difference:is indicated at 224 of Figure 12(D), and: is proportional to two times the phase difference ~R
produced by rotation of the optical loop.
Referring now to Figure 13, graph line 250 illustrates t ~ the phase shi:Et detected by phase meter 174 between the channel 1 and channel 2 filter output signals as compared to t~e actual Sagnac phase shift~ It is .noted that a substantially linear result is achieved over a very wide `: dynamic range. The dots alono line 251) identify pLots from particular e~perimental measurements which were m ad e . :

,, From the above, it is seen that the simpLe, open-loop rotation sensor of Figure 1 n comprises a device which ~-~ implements an important technique for utilizing an . open-loop rotation sensor to achieve extended dynamic sensin,g of fiber optic gyroscope rotation. The device :~ accomplishes this accurately, with a high degree of ~ sensitivity, and without use of extensive electronic :: components or other devices which have been necessary in ~:~ o ther types of rotation sensors .
Another preferred embodiment of the rotation sensor of tlte present invention i9 illustrated in Figure 14. In this embodiment, switch 156 of Figure ln is replaced by a conventional electronic gate 300. Gate 30n functions in ~ response to a signal received on line 164 from a delay .: 15 circuit 162 in the manner previously described with : respect to the embodiment of Figure 10. Thus, ~ate 300 produces square wave amplitude modul~tion of a signal .:: received from amplifier 152 in accordance with the device illustrated in Figure 10. When modulated at the ~ 20 appropriate phase and amplitude with respect to frequency : ~ fm~ the amplitude modulated si~nal of this embodiment is defined by the equation cos (n~mt ~ ~ This corresponds with the definition in equation (17) for I1 of channel 1 of the embodiment of Figure 1 0O
: From ~iate 300, the amplitude modulated signal is transmitted via line 302 to a band pass filter 3()4 which is tuned to a selected harmonic of the phase modulation frequency fm . ~ For reasons discussed below~ in the specific embodiment :illustrated in Figure 14, the 3~ :frequency selected should be the first harTionic, corresponding to the fm frequency. The filtered signal is then transmi:tted via line 3û6 to a phase meter 308, which corresponds in function to meter 174 in the embodiment of Figure 10. The si,~nal from line 3()6 is compared with a reference signal received via line 310 from deLay circuit 16:~ at frequency fm~ The signal on line 31 l corresponds :~ .

' ~ ' ~ ' : ' . , ' ' , ' ' - .
.. .. : .

to the phase di:Eference modulation signal as delayed in circuit 162, which is defined by the term cos ~mt. The resulting output from the phase meter 3n8 corresponds to the phase difference si~nal ~R produced by rotation of the optical loop.
If it is desired to select a harmonic other than the first in hand pass filter 3n4, the device of Figure 14 may ~ be modified by including a frequency multiplier (not :~: : shown) in line 310, so that the selected harmonic of the ; 10 fm phase modulation si~nal can be applied to the appropriate input of phase meter 308. As with the output from phase meter 174 of the embodiment of Figure tO, the output of phase meter 3~8 can be utilized by conventional . digital devices for indicating the rotation rate of the optical loop.
Referring to Figure 15, operation of the device of:
Figure 14 may be graphically described. Specifically, when a phase modulation signal is applied from signal:
: generator 1 sn to phase modulator 38 at frequency fm~ the ~ resulting phase difference modulation waveform..:at frequency fm is illustrated at 350. At this frequency, :~ gate 3~0 provides square wave amplitude modulation in accordance with the waveform illustrated at 352.
: : Figure~ 15(B) illustrates the waveshapes produced when z~ 25 : the optical loop is not rotated~ ~pecifically,~when no:
rotation~:is experienced in the loop, the waveshape of the ::output æignal of de~ector:30 is illustrated at :354. The : :: amplitude:~modulated:output signal at frequency f~ produced .by gate ~3~0~ L S ~illustrated at 35fi, with the resulting~
~: ~ output of~band:pass;filter 304 at frequency fm illustrated at~35~8. Under these conditionsj the phase m.eter 308 will :: detect the::time inte:rval between zero crossing of the :waveshape 35~ and the leading ed~e of the reference signal : 352. Since~ like sinusoidal terms cancel in this : 35 condition, the phase meter 308 indicates that the rotation .

. :
.
.

:, ................ .. . .
': ' ' ~

~7~

rate i9 zero. This situation is graphical]y illustrated at 360 in Figure 15(~).
` Figure 15(C) illustrates the situation which exists in the device of Figure 14 durin& rotation of the optical -~ 5 loop. In this condition, with phase modulation at frequency fm and gated amplitude modulation at frequency -;~ fm the output of detector 3n is illustrated at 362.
` Accordingly, the amplitude modulated signal from gate 300 is illustrated at 364. The corresponding waveshape as seen on the output of band pass filter 304 at frequency f~
is illustrated at 366, In this situation, it is noted that the zero crossin~ of waveshape 36h is oEfset rom the correspondin~ leading edge of the reference si~nal 352.
Accordin~ly, this offset is detected in phase meter 308 and an output signal is~ produced correspondin~ to the phase difference ~signaI produced in ~ the counter-propagating waves by the-rotation of the optical loop. The amount of this phase shift, corresponding to the ~, is illustrated at 36~ of Figure 15~C).
~ ~ Opti~onally,~gating of the device of either of Figures or 14 could be accomplished optically ratlier than electrically by utilizing at least one optical gate, such as a shutter, positioned at any desired location on the optical loop prior to p~otodetector 30. For example, the optical gate could be positioned at the input of the light~
source, between the first directional coupler and the laser di;ode. ~In such a configuration, the ~ate would continue to be controll~ed by a delay si~nal at a frequency fm ~so that the ~light~traveling within the loop would be amplitude modulated at the;fm frequency. In all other re;spects, ~he use of~optical gating would pro~ide a result su~stantially identical to that described with respect to the de~ice of Figures 1~ or 14.
In summary, not only does the invention described ,~ ~
herein comprise a significant imyrovement over the prior art in de~ecting rotation rates of optical gyroscopesj but '''' '~; :

. .

'' '. ' ' - ' ' ' ;-''',' ' .. : .. ' - ' ,, ' ' ~ ~2i7~

it also overcomes other long-existin~ proble~s in th~ ar~
by (1) providing an open-loop sensor havin~ a substantially unli~ited dynamic range; ~2) providinv such a sensor which is compatible with conventional electronic ~ 5 and fiber optic devices; (3) providin~ a system which is - simple in construction, and does not require the co~plex 1~ electronic feedback systems or other control devices whichare co~monly utilized in other types of interferometers;
and (4) provides very accurate results which are directly ln useable by ~i~ital devices, In addition to overcoming these problems, devices of the type described herein are very inexpensive to produce as co~pared to the other sensin~ devices currently on the market, and thus the invention provides ~reat economic savin~s in conjunction with its use in commercial applications. 8ecause of its si~plicity and e~tended dynamic ran~e, as well as its mini~al space require~ents, the device and method described herein finds application in many and varied uses, and can be easily incorporated into many different tv~es of embodiments.
, Although the present invention has been described with reference to a Sa~nac interfero~eter, it will be ;~
appreciated that the ~etection syste~ of the present invention is equally applicable for all other types of inter'erometers, such as 'lach-Zehnder interfero~eters, Michelson interferometers, and Fabrav-Pero~
inter'erometers. A'l of the above-identified interferom~eters are well-known in the art and provi~e an interferometer out~u~ si~nal comprised of two interferin~
light waves, ~wherein the phase difference between the ht waves derer~i~es the intensity of the out?ut si~al. Further, t'~e i~ention is applicahle for the type of fiber optic interferometer disclosed in U.S. Patent No. 4,469,397 issuea September 4th, 1984 and entitled "Fiber Optic Resonator", which is analogous to a 11 Fabray-Perot interferometer.
' ~ : -~7 Ei~
.

It will also be appreciated that, while the invention has been described in terms of a fiber optic interferometer, it is equally applicable to interferometers using bulk optic components, such as beam splitters and/or mirrors. Those skilled in the art will understand that, if optical fibers are not used to guide the interfering waves, modulation of the waves may be accomplished by other means, such as mirrors or ~ electro-optical devices.
`' ~ 10 While the preferred embodiment was described in a rotation sensin~ context, the invention is equally applicable to any interferometer application which produces a phase difference between two light waves. The present invention is capable of detecting such phase difference regardless of the particular quantity or phenomenon which produces the phase difference. Thus, the invention is appropriate for use with any type of interferometer, regardless of the structural aspects of the interferometer, the components used to construct such interferometer, or the quantity which produces the detected phase difference between the two interfering waves of the interferometer.
The invention may be embodied in other specific forms without departin~ ~from its spirit or essential ~characteristics. The described embodiments are tb be considered in all respects only as illustrative and not ; restrictive. The scope of the invention isj therefore, indicated by the appended claims rather than by the foregoin~ description. All chan~es which come within the meanin~and~ range of equivalency of the cIaims are to he~
embraced~ wlthin their scope.

": :
:~:": : :

.
~; .

Claims (43)

1. In a method of detecting shifts in phase difference of interfering light waves which propagate within an interferometer formed of optical fiber, and the interferometer being of the type which produces an optical output signal comprised of the interfering light waves, wherein the phase difference between the interfering light waves is dependent upon the respective optical path lengths traveled by the light waves within the interferometer and upon the influence of external forces applied to the interferometer, and wherein the intensity of the optical output signal is dependent upon the phase difference of the interfering light waves, the improvement comprising the steps of:
combining the interfering light waves to form an output having a waveform which corresponds to the phase difference of said light waves;
passing the output through an amplitude modulator to mix the output waveform with a modulating waveform which is at a modulation frequency; and selecting a predetermined frequency from the amplitude modulated output to produce a signal which is representative of shifts in said phase difference, wherein said predetermined frequency is a harmonic of the modulation frequency.

2. A method of detecting shifts in phase difference of interfering light waves as defined in Claim 1, wherein the step passing the output through an amplitude modulator comprises the step of amplitude modulating the output to provide a low frequency signal having a phase shift which corresponds to the shift in the phase difference of the interfering light waves.

3. A method of detecting shifts in phase difference of interfaring light waves as defined in Claim 1, wherein the method further comprises the step of phase modulating the interfering light waves at the modulation frequency;
and wherein the step of passing the output through an amplitude modulator comprises the step of mixing the output waveform with the modulating waveform at the modulation frequency to produce a signal having harmonics of said modulation frequency which contain both cosine and sine components of the output.

4. A method of detecting shift in phase difference of interfering light waves as defined in Claim 3 wherein the step of selecting a predetermined frequency comprises the step of detecting a component of the amplitude modulated output which is at a harmonic of the modulation frequency; and wherein the method further comprises the step of comparing the detected component with a reference signal to produce a signal representative of shifts in said phase difference.

5. A method of detecting shifts in phase difference of interfering light waves as defined in Claim 1 wherein the shifts in the phase difference of the interfering waves are caused by external forces applied to the interferometer and wherein the method further comprises the step of measuring the signal representative of shifts in the phase difference to determine the response of the interferometer to the external forces.

6. A method of detecting shifts in phase difference as defined in Claim 1, further comprising the step of passing components of the signal at said selected frequency from the amplitude modulator through a comparator to produce a signal which is proportional to shifts in said phase difference caused by influence of external forces upon the interferometer.

7. In a method of detecting shifts in phase difference of interfering light waves which propagate within an interferometer of the type which provides for production of an optical output signal comprised of the interfering light waves, wherein the phase difference between the interfering light waves is dependent upon the respective optical path lengths traveled by the light waves within the interferometer and upon the influence of external forces applied to the interferometer, and wherein the intensity of the optical output signal is dependent upon the phase difference of the interfering light waves, the improvement comprising the steps of:
phase modulating the interfering light waves at a modulation frequency;
combining the interfering light waves to form an output having a waveform which corresponds to the phase difference of said light waves;
mixing the output waveform with a modulating waveform at the modulation frequency to produce a modulated output having harmonics of said modulation frequency which contain both cosine and sine components of the output;
alternately placing the modulated output on one of two channels at the selected frequency, so as to define amplitude modulated signals in the channels which are substantially 180° out of phase with each other; and detecting elected components of the modulated signals in each channel to produce signals whose phase difference corresponds to the shift in phase difference of the interfering light waves.

8. A method of detecting shifts in phase difference of interfering light waves as defined in Claim 7, further comprising the steps of:
comparing the detected components from each channel to identify their phase difference; and providing a signal which corresponds to the phase difference of the detected components, and which is representative of the shifts in phase difference of interfering light waves.

9. In an interferometer formed of optical fiber, and the interferometer being of the type which produces an optical output signal comprised of interfering light waves which propagate within the interferometer, wherein the phase difference between the interfering waves is dependent upon the respective optical path lengths traveled by the light waves within the interferometer and upon the influence of external forces applied to the interferometer, and wherein the intensity of the optical output signal is dependent upon the phase difference of the interfering light waves, an apparatus for detecting shifts in phase difference of the interfering light waves comprising:
means for combining the interfering light wave to form an output having a waveform which corresponds to the phase difference of said light waves;
means for mixing the output from the combining means with a modulating waveform which is at a modulation frequency to provide an amplitude modulated output; and means for selecting a predetermined frequency from the amplitude modulated signal to produce a signal which is representative of shifts in said phase difference, and wherein said predetermined frequency is a harmonic of the modulation frequency.

10. An apparatus for detecting shifts in phase difference as defined in claim 9 further comprising:
a signal generator for providing a phase modulation signal at a selected modulation frequency;
a phase modulator, responsive to the signal generator for phase modulating the interfering light waves in the interferometer at the selected modulation frequency; and wherein the means for mixing functions to amplitude modulate the output at the selected modulation frequency.

11. An apparatus for detecting shifts in phase difference as defined in Claim 10, wherein the means for selecting comprises a device for detecting a component of the amplitude modulated output which is at a harmonic of the selected modulation frequency: and wherein the apparatus further comprises a device for comparing the detected component with a reference signal to produce a signal which corresponds to the phase difference between the detected component and the reference signal, and which is proportional to the shifts in phase difference of the interfering light waves.

12. An apparatus for detecting shifts in phase difference as defined in Claim 9 wherein the shifts in the phase difference of the interfering waves are caused by external forces applied to the interferometer and wherein the signal which is representative of shifts in the phase difference provides an indication of the response of the interferometer to the external forces.

13. In an interferometer of the type which provides for production of an optical output signal comprised of interfering light waves which propagate within the interferometer, wherein the phase difference between the interfering waves is dependent upon the respective optical path lengths traveled by the light waves within the interferometer and upon the influence of external forces applied to the interferometer, and wherein the intensity of the optical output signal is dependent upon the phase difference of the interfering light waves, an apparatus for detecting shifts in phase differences of the interfering light waves comprising:
means for combining the interfering light waves to form an output having a waveform which corresponds to the phase difference of said light waves;
a first device for alternately placing the amplitude modulated output signal on one of two device outputs at a modulation frequency, so as to define amplitude modulated signals on the outputs which are substantially 180° out of phase with each other; and means for selecting a predetermined frequency from the amplitude modulated signal to produce a signal which is representative of shifts in said phase difference.

14. An apparatus for detecting shifts in phase difference as defined in Claim 11 wherein the means for selecting comprises:
at least one second device for detecting selected components of the amplitude modulated signals on each of the first device outputs, and for providing signals representative of the selected components, wherein the phase difference of the representative signals corresponds to the phase difference caused by application of the external forces to the interferometer; and a third device for comparing the signals representative of the selected components and providing an output signal which corresponds to the phase difference of said representative signals, said output signal being proportional to the shifts in phase difference of the interfering light waves.

15. A method of detecting the rotation rate of an optical loop formed of optical fibers having counter-propagating light waves therein whose phase difference is shifted by the rotation rate of the optical loop, comprising the steps of:
phase modulating the counter-propagating waves at a selected frequency;
combining the counter propagating waves to produce an output signal;
passing the output signal through an amplitude modulator to mix the output signal with a modulating waveform substantially at the selected frequency to transpose the shift in the phase difference into a phase shift in a low frequency signal;
monitoring a selected component of the low frequency signal to identify shifts in the counter-propagating wave phase difference caused by rotation, wherein said selected component is a harmonic of the selected frequency; and providing an output signal proportional to the rotation rate of the optical loop.

16. A method of detecting the rotation rate of an optical loop as defined in Claim 15, wherein the step of passing the output signal through an amplitude modulator comprises the step of amplitude modulating the output signal to produce a signal having harmonics of the selected frequency which contain both cosine and sine components of the output signal.

17. A method of detecting rotation rake of an optical loop having counter-propagating light waves therein whose phase difference is shifted by the rotation rate of the optical loop, comprising the steps of:
phase modulating the counter-propagating waves at a selected frequency;
combining the counter-propagating waves to produce an output signal;
alternately transmitting the output signal to one, and then the other, of two outputs on a switch such that the signals on said switch outputs are out of phase with each other;
filtering the signals from the switch outputs so as to provide filtered signals comprising selected components which are substantially harmonics of said signals from the switch outputs, said filtered signals comprising amplitude modulated signals at the frequencies of the selected components, and wherein the combined, filtered signals comprise a phase shift in a low frequency signal;

monitoring selected components of the low frequency signal to identify shifts in the counter-propagating wave phase difference caused by rotation;
and providing an output signal proportional to the rotation rate of the optical loop.

18. A method of detecting the rotation rate of an optical loop as defined in Claim 17, wherein the steps of monitoring selected components of the low frequency signal and providing an output signal comprise the steps of:
comparing the phases of the filtered signals; and providing an output signal which corresponds to the phase difference of the filtered signals and which is proportional to the rotation rate of the optical loop.

19. A method of detecting the rotation rate of an optical loop as defined in Claim 17, wherein the step of alternately transmitting the output signal comprises the step of square wave modulating the output signal.

20. A method of detecting the rotation rate of an optical loop as defined in Claim 12, wherein the signals on the switch outputs are substantially 180° out of phase with each other.

21. A method of detecting rotation rate of an optical loop having counter-propagating light waves therein whose phase difference is shifted by the rotation rate of the optical loop, comprising the steps of:
phase modulating the counter-propagating waves at a selected frequency;
combining the counter-propagating waves at a selected frequency:
gating a portion of the output signal onto a gate output so as to provide an amplitude modulated signal on the gate output;
filtering the signal from the gate output so as to provide a filtered signal comprising a selected harmonic component of the signal from the gate output, said filtered signal thus comprising an amplitude modulated signal at the frequency of the harmonic component, said filtered signal having a phase shift corresponding to that of the phase difference caused by loop rotation;
monitoring selected components of the low frequency signal to identify shifts in the counter-propagating wave phase difference caused by rotation;
and providing an output signal proportional to the rotation rate of the optical loop.

22. A method of detecting the rotation rate of an optical loop as defined in Claim 21, wherein the steps of monitoring selected components of the low frequency signal and providing an output signal comprise the steps of:
comparing the filtered signal with a reference signal at substantially a harmonic of the first selected frequency; and providing an output signal which corresponds to the phase difference between the filtered signal and the reference signal, and which is proportional to the rotation rate of the optical loop.

23. An apparatus for detecting rotation rate of any optical loop formed of optical fibers wherein light waves may be counter-propagated, the phase difference of the light waves being shifted in response to rotation of the optical loop and the light waves being combined to form an optical output signal, the apparatus comprising:
a signal source for providing a phase modulation signal at a first selected frequency;
a phase modulator responsive to the modulation signal for phase modulating the counter-propagating waves at the first selected frequency;
a detector for sensing the optical output signal and for providing a corresponding electrical output signal;
an amplitude modulator circuit electrically connected to the output of the detector and responsive to the signal generator for mixing the output signal waveform with a modulating waveform to provide an amplitude modulated output signal at a second selected frequency which is a harmonic of the first selected frequency; and a phase sensitive device for monitoring the phase of the amplitude modulated output signal at the second selected frequency in order to detect the phase shift in said output signal caused by rotation of the optical loop.

24. An apparatus for detecting rotation rate of an optical loop as defined in Claim 23, further comprising means for substantially removing any direct current component from the electrical output signal, thereby providing a substantially alternating current signal to the amplitude modulator circuit.

25. An apparatus for detecting rotation rate of an optical loop wherein light waves may be counter-propagated, the phase difference of the light waves being shifted in response to rotation of the optical loop and the light waves being combined to form an optical output signal, the apparatus comprising:
a signal source for providing a phase modulation signal at a first selected frequency;
a phase modulator responsive to the modulation signal for phase modulating the counter-propagating waves at the first selected frequency;
a detector for sensing the optical output signal and for providing a corresponding electrical output signal;
a switch which alternately transmits the electrical output signal to one and then the other of two outputs on the switch such that the signals on the switch outputs are out of phase with each other;
at least one filter responsive to the signals from the outputs of the switch for detecting selected harmonic components of the switch output signals and providing filtered signals corresponding to the selected components, said filtered signals comprising an amplitude modulated output signal at a second selected frequency; and a phase sensitive device for monitoring the phase of the amplitude modulated output signal at the second selected frequency in order to detect the phase shift in said output signal caused by rotation of the optical loop.

26. An apparatus for detecting rotation rate of an optical loop as defined in Claim 25, wherein the phase sensitive device comprises a phase detector for comparing the phases of the filtered signals, and for providing an output signal which corresponds to the phase difference of the filtered signals and is proportional to the rotation rate of the optical loop.

27. An apparatus for detecting rotation rate of an optical loop as defined in Claim 25, wherein the signals on the switch output are substantially 180° out of phase with each other.

28. An apparatus for detecting rotation rate of an optical loop as defined in Claim 25, wherein the switch functions to provide square wave amplitude modulation of the electrical output signal.

29. An apparatus for detecting rotation rate of an optical loop wherein light waves may be counter-propagated, the phase difference of the light waves being shifted in response to rotation of the optical loop and the light waves being combined to form an optical output signal, the apparatus comprising:

a signal source for providing a phase modulation signal at a first selected frequency;
a phase modulator responsive to the modulation signal for phase modulating the counter-propagating waves at the first selected frequency;
a detector for sensing the optical output signal and for providing a corresponding electrical output signal;
a gate which functions to mix the output signal waveform with a modulating waveform and to thereby pass a portion of an amplitude modulated to a gate output;
a filter responsive to the amplitude modulated signal for detecting selected harmonic components of said signal, so as to provide a filtered signal whose phase shift corresponds to the phase shift produced in the counter propagating waves by the rotation rate;
and a phase sensitive device for monitoring the phase of the filtered signal in order to detect the phase shift in said output signal caused by rotation of the optical loop.

30. An apparatus for detecting rotation rate of an optical loop as defined in Claim 29, wherein the phase sensitive device compares the phase of the filtered signal with a reference signal at the frequency of the selected harmonic components, and wherein the phase sensitive device provides an output signal which corresponds to the phase difference between the filtered signal and the reference signal, and which is proportional to the rotation rate of the optical loop.

31. In a method of detecting the effect of external forces on an interferometer of the type formed of optical fiber and which provides for production of an optical output signal comprised of interfering light waves which propagate within the interferometer, wherein the phase difference between the interfering light waves is dependent upon the respective optical path lengths traveled by the light waves within the interferometer and upon the influence of external forces applied to the interferometer and wherein the intensity of the optical output signal is dependent upon the phase difference of the interfering light waves, the improvement comprising the steps of:
combining the interfering light waves to form an output having a waveform which corresponds to the phase difference of said light waves:
mixing the output waveform with a modulating waveform at a modulation frequency to provide a low frequency signal which is at a harmonic of the modulation frequency, and whose phase shift corresponds to the shift in said phase difference of the light waves, which shift is induced by external forces applied to the interferometer;
monitoring selected components of the low frequency signal to identify said phase shift of the low frequency signal; and providing an output signal corresponding to the phase shift of the low frequency signal and representative of the response of the interferometer to the external forces.

32. A method of detecting the effect of external forces on an interferometer as defined in Claim 31, wherein the step of mixing the output waveform comprises the steps of:
phase modulating the interfering light waves at a selected frequency; and passing the output through an amplitude modulator at the selected frequency to produce a signal having harmonics of the selected frequency which contain both cosine and sine components of the output.

33. A method of detecting the effect of external forces on an interferometer as defined in claim 32, wherein the step of providing an output signal comprises the steps of:
detecting a component of the amplitude modulated output which is at a harmonic of the selected frequency; and comparing the detected component to a reference signal to produce a signal corresponding to the phase shift of the detected component, and representative of the response of the interferometer to the external forces.

34. In a method of detecting the effect of external forces on an interferometer of the type which provides for production of an optical output signal comprised of interfering light waves which propagate within the interferometer, wherein the phase difference between the interfering light waves is dependent upon the respective optical path lengths raveled by the light waves within the interferometer and upon the influence of external forces applied to the interferometer and wherein the intensity of the optical output signal is dependent upon the phase difference of the interfering light waves, the improvement comprising the steps of:
phase modulating the interfering light waves at a selected frequency;
combining the interfering light waves to form an output having a waveform which corresponds to the phase difference of said light waves;
alternately placing the output on one of two channels at the selected frequency; so as to define amplitude modulated signals in the channels which are substantially 180° out of phase with each other;
detecting selected components of the modulated signals in each channel to produce signals whose phase difference corresponds to the phase difference caused by application of the external forces to the interferometer; and providing an output signal corresponding to the phase difference of the selected components and representative of the response of the interferometer to the external forces.

35. A method of detecting the effect of external forces on an interferometer as defined in Claim 34, wherein the step of providing an output signal comprises the steps of:
comparing the detected components from each channel to identify their phase difference; and providing a signal corresponding to the phase difference of the detected components, and representative of the response of the interferometer to the external forces.

36. A method of detecting rotation rate of an optical loop formed of optical fiber having counter-propagating light waves therein whose phase difference is shifted by the rotation rate of the optical loop, comprising the steps of:
combining the counter-propagating light waves to form an output having a waveform which corresponds to the phase difference of said light waves:
passing the output through an amplitude modulator to mix the output waveform with a modulating waveform at a modulation frequency;
selecting a predetermined frequency from the amplitude modulated output to produce a signal which is representative of shifts in said phase difference, wherein said predetermined frequency is a harmonic of the modulation frequency; and measuring the phase of at least one component of said signal which is representative of shifts, to determine the rotation rate of the optical loop.

37. A method of detecting rotation rate as defined in Claim 36 wherein the step of measuring the phase of at least one component comprises the step of comparing a selected component to a reference signal to produce a signal which is proportional to shifts in the counter-propagating light wave phase difference.

38. A method of detecting rotation rate as defined in Claim 37 wherein the step of comparing a selected component to a reference signal comprises the step of measuring elapsed time between detection of an edge of a waveform of the reference signal and a zero crossing of the selected component waveshape on a horizontal axis to obtain the value which is proportional to shifts in the counter-propagating light wave phase difference.

39. A method of detecting rotation rate as defined in Claim 36 wherein the step of measuring the phase of at least one component comprises the step of comparing a first selected component to a second selected component to produce a signal proportional to shifts in the counter-propagating light wave phase difference.

40. A method of detecting rotation rate as defined in Claim 39 wherein the step of comparing a first selected component to a second component comprises the step of measuring elapsed time between a zero crossing of the first selected component waveform and a zero crossing of the second selected component waveform on a horizontal axis to obtain the value which is proportional to shifts in the counter-propagating light wave phase difference.

41. An apparatus for detecting rotation rate of an optical loop wherein light waves may be counter propagated, the phase difference of the light waves being shifted in response to rotation of the optical loop and the light waves being combined to form an optical output signal, the apparatus comprising:
a signal source for providing a phase modulation signal at a first selected frequency;
a phase modulator responsive to the modulation signal for phase modulating the counter-propagating waves at the first selected frequency;

a detector for sensing the optical output signal and for providing a corresponding electrical output signal;
an amplitude modulator circuit electrically connected to the output of the detector and responsive to the signal generator for mixing the output signal waveform with a modulating waveform to provide an amplitude modulated output signal at a second selected frequency;
a delay circuit interposed between the amplitude modulator circuit and the signal source for delaying signals transmitted from the signal source to the amplitude modulator circuit so as to synchronize operation of the amplitude modulator circuit with the waveform of the electrical output signal; and a phase sensitive device for monitoring the phase of the amplitude modulated output signal at the second selected frequency in order to detect the phase shift in said output signal caused by rotation of the optical loop.

42. A method of detecting rotation rate of an optical loop having counter-propagating light waves therein whose phase difference is shifted by the rotation rate of the optical loop , comprising the steps of:
combining the counter-propagating light waves to form an output having a waveform which corresponds to the phase difference of said light waves;
passing the output through an amplitude modulator to mix the output waveform with a modulating waveform;
selecting a predetermined frequency from the amplitude modulated output to produce a signal which is representative of shifts in said phase difference;
and measuring elapsed time between detection of an edge of a waveform of the reference signal and a zero crossing of the selected component waveshape across a horizontal axis to obtain a value which is proportional to shifts in the counter-propagating light wave phase difference.

43. A method of detecting rotation rate of an optical loop having counter-propagating light waves therein whose phase difference is shifted by the rotation rate of the optical loop, comprising the steps of:
combining the counter-propagating light waves to form an output having a waveform which corresponds to the phase difference of said light waves;
passing the output through an amplitude modulator to mix the output waveform with a modulating waveform selecting a predetermined frequency from the amplitude modulated output to produce a signal which is representative of shifts in said phase difference;
and measuring elapsed time between a zero crossing of the first selected component waveform and a zero crossing of the second selected component waveform on a horizontal axis to obtain the value which is proportional to shifts in the counter-propagating light wave phase difference.